Black Caps captain Brendon McCullum believes if the right balance between endeavour and execution is achieved at Port Elizabeth tomorrow, New Zealand could be celebrating an unlikely T20 series success against South Africa.

''We knew we could be competitive in this series,'' McCullum said after the stunning form reversal at East London on Monday where New Zealand snatched a thrilling last-ball victory on the back of an exhilarating Martin Guptill century.

''We've got everything to play for [and we need to] make sure we take another smart, but aggressive, approach in the next game,'' McCullum said.

Meanwhile, almost more amazing than the fact that New Zealand beat South Africa, was the way Guptill celebrated it.

Not a noted expresser of emotion, Guptill went right off, after hitting the last ball of the match from Rory Kleinveldt through cover for four.

In doing so, he scored his first international T20 hundred and got the Black Caps up by eight wickets.

Afterwards, Guptill described his 69-ball knock of 101 not out as the best innings of his Twenty20 career.

''It was a very special moment and I'm proud to have done it,'' Guptill said.

Which makes you wonder why he seemed so angry at the time, uttering oaths at the sky and generally behaving like an embittered premier league footballer.

All that was missing was Guptill pulling his playing shirt over his head to reveal some slogan emblazoned on a garment beneath.

New Zealand needed 169 to beat South Africa, with the match reduced to 19-overs per side due to a floodlight failure during the Proteas' innings.

Having lost by eight wickets in the first match of the three-game series, you could understand why Guptill and his team-mates were so pumped up at winning.

But his was still a curious way to react to a moment of personal, and team, success.

Guptill, having quickly reverted to his usual placid demeanour, said his hundred at East London didn't mean he was well-placed to get more runs in the decider.

''We'll just have to see how we feel on Boxing Day and what the pitch is like,'' he said.

The Black Caps recklessly threw away wickets in the first match of the series in Durban.

Bowled out for just 86, New Zealand were little better in the field.

Notwithstanding the brilliant running catch James Neesham took at East London, there was still some untidiness about New Zealand's fielding.